The complications of placental insufficiency, pre-eclampsia and fetal growth restriction (FGR) are major causes of perinatal as well as maternal morbidity and mortality. In normal pregnancy, impedance to flow in the uterine arteries decreases with gestation, and histopathological studies suggest that this is due to trophoblastic invasion of the spiral arteries and their conversion into low-resistance vessels. Failure of trophoblastic invasion is associated with complications of uteroplacental insufficiency. Several Doppler screening studies, both in the second and more recently in the first trimester of pregnancy, have demonstrated an association between increased impedance to flow in the uterine arteries and subsequent development of pre-eclampsia, FGR and perinatal death.